In the continuous casting of steel, molten steel is poured from a ladle into a tundish from whence the molten steel passes to the continuous casting mold. There is an increasing demand from customers of continuous casters for improved steel cleanliness and more attention is being paid by steel companies to the factors which influence the quantity of non-metallic inclusions in steel.
Deoxidation of the steel usually is effected in the ladle and some deoxidation products are removed by way of ladle slag. However, reoxidation of the steel and dissolved alloys may occur in the ladle, tundish and the continuous casting mold or during one of the transfer operations, thus adversely affecting the final cleanliness in the steel.
There exists, therefore, a need for a flux for both the ladle and the tundish. An ideal ladle flux material and an ideal tundish flux material should have the following functions:
(a) Thermal insulation of the steel so as to avoid premature freezing and excessive skulling within the ladle or the tundish,
(b) Protection of the steel from atmospheric oxidation, and
(c) Absorption of inclusions reaching the flux-metal interface.
As is discussed in more detail below, these functions are also required of mold fluxes used in the continuous casting mold but there are other more important properties also demanded of mold fluxes which are not required of a ladle flux or a tundish flux. The better the ladle flux and the tundish flux perform in all three respects, the lesser is the burden imposed on the mold flux in the subsequent continuous casting procedure and hence the better the mold flux is able to function.
The first function of the ladle flux and the tundish flux requires an outer solid particulate layer since heat transfer through a completely liquid layer is quite rapid, while the other two functions require a liquid layer in contact with the molten steel. Relatively little attention has been given to the third requirement but as ladle designs and tundish designs are improved and flow control devices are employed in the ladle and tundish, the extent of inclusion separation is expected to increase, especially if a suitable flux composition can be provided which will absorb the non-metallic reaction products.
It is becoming increasingly standard practice to employ flux material in either the ladle or the tundish or both. Calcium aluminate is commonly used as a flux material in the ladle. Rice hulls usually are used as a flux material in the tundish. Rice hulls, which contain approximately 85% silica, provide very good thermal insulation and are employed mainly for that reason, but are not very effective for oxidation protection or inclusion absorption. Rice hulls are solid at steel-casting temperatures, so that any extent to which the material fulfils the second and third requirements results from fluxing of silica by other oxides, probably iron and manganese oxides produced by reoxidation, to form a liquid layer, a somewhat unsatisfactory condition.
Various mold fluxes also have been used as tundish fluxes. These materials generally are not very effective in terms of thermal insulation, unless a powder layer is maintained, although they usually provide reasonably effective inclusion absorption and protection from oxidation. These materials are formulated to function specifically as mold fluxes and therein lies their drawback as potential tundish fluxes. In addition to the three properties mentioned above, mold fluxes also are required to infiltrate between the solidifying steel and the mold wall in the mold. This characteristic, which is the most important one for a mold flux, much more so than the three functions mentioned above, assists in controlling the rate of heat transfer from the solidifying steel to the mold wall and also results in a lubricant for the solidifying steel as it passes through the mold. In order to provide the flow property, the viscosity of the mold flux at steel-casting temperatures is generally low, usually as a result of the presence of fluidizers, such as sodium oxide, sodium fluoride or calcium fluoride. Carbon also usually is added to control the rate of melting of the mold flux. As a result, the liquidus temperature of mold fluxes is generally in the range of about 1000.degree. to about 1150.degree. C.
Both the low viscosity and the presence of fluidizers are detrimental in both the ladle and the tundish environments, however, since, for example, vortexing in the ladle or tundish as the liquid steel passes to the continuous casting mold can readily result in gross entrainment of the low viscosity material and the fluidizers can cause serious erosion of the refractory material generally used in tundish parts, such as the lining, weirs and ceramic shrouds where submerged nozzles are in use. In addition, as a result of their complex formulation, mold fluxes tend to be rather expensive.
Neither rice hulls nor mold fluxes, therefore, perform satisfactorily in the ladle or tundish since neither possess all the properties necessary to function ideally without introducing additional problems. Similarly, calcium aluminate can be unsatisfactory as a flux due to its limited capacity to absorb alumina and its ability to transfer hydrogen from moist air to molten steel. This effect is undesirable and any tundish or ladle flux should not enhance the transfer of undesirable elements, such as hydrogen from atmospheric moisture, as well as from the flux material itself to the molten steel.
The optimum practical composition for a ladle flux and a tundish flux has a somewhat narrow range of properties, as will be seen from the following discussion. Since a liquid layer is desired in contact with the molten steel, which typically has a temperature in the range of about 1475.degree. to about 1600.degree. C. in both the ladle and the tundish, the flux should be completely molten around 1,450.degree. C., but since too low a viscosity causes problems, as discussed above, the superheat should not be large, so that a liquidus temperature of between 1,350.degree. C. and 1,450.degree. C. is appropriate, a temperature well above that for most mold fluxes. Significant quantities of iron oxides, in the form of FeO or Fe.sub.2 O.sub.3, or manganese oxide are undesirable since they introduce oxygen into the steel and may ultimately lead to non-metallic inclusions in the solidified product. Further, since one of the main functions of the flux is to absorb inclusions and since most continuously-cast steel slabs are aluminum killed so that the inclusions are alumina or calcium aluminates, the initial alumina content of the flux should be as low as possible, so as to promote inclusion absorption. As previously observed, the quantities of fluidizers, such as sodium oxide, sodium fluoride and calcium fluoride, permitted are severely limited by the melting range, viscosity and refractory erosion considerations. The only oxide left to combine with lime to produce a flux with reasonable melting characteristics is silica.